Complex fluoro salt cementation method for coating refractory metallic substrates



Aug. 8, 1967 J. ZUPAN 3,335,028

COMPLEX FLUORO SALT CEMENTATION METHOD FOR COATING REFRACTORY METALLIC SUBSTRATES Filed Nov. 26, 1963 REACTION MIXTURE COLUMBIUM SUBSTRATE COMPLEX BI-METAL FLUORIDE SALT,

REACTION COATING METAL POWDER ENCLOSE IN ENVIRONMENT AND HEAT TO I800-2300F ENVIRONMENT FREED OF OXYGEN, NITROGEN 8x HYDROGEN SUBSTRATE COATED FIG. 1

INVENTOR.

JAN EZ ZUPAN ATTORNEY United States Patent COMPLEX FLUORO SALT CEMENTATION METH- OD FOR COATING REFRACTORY METALLIC SUBSTRATES Janez Zupan, Rochester, N.Y., assignor to Ritter Pfaudler Corporation, a corporation of New York Filed Nov. 26, 1963, Ser. No. 326,218 8 Claims. (Cl. 117107.2)

ABSTRACT OF THE DISCLOSURE at 1800 F. to 2300 F.

This invention relates to cementation coating processes for the refractory metals and, in particular, to single or multiple cycle processes for vapor plating columbium or niobium, one object of this invention being the provision of a more satisfactory process of this nature.

Columbium has come into increasing use for structural components for use at elevated temperatures. It has proven very satisfactory for this purpose because of its high melting point, its great strength-to-weight ratio at and above 2600 F., its superior workability when compared to molybdenum and other refractory metals useable at temperatures in this range. However, many of these applications for such refractory metals require resistance to oxidation and columbium oxidizes rapidly in contact with the air at the temperatures mentioned above. Some improvement in the oxidation resistance of columbium has been attained by alloying but the results achieved by this method have so far proven to be unsatisfactory for any extended use at high temperature in contact with air or the oxidizing atmosphere. For this reason, it is desirable to provide an oxidation resistant coating which can be applied to the surface of columbium and columbium alloys which will prevent oxidation of such alloys at elevated temperatures. The provision of such a coating is a primary object of this invention. I

Much work has been done in the pastin an attempt to provide such oxidation resistant coatings for columbium. Mainly, the processes were designed to provide silicide or modified silicide type coatings for the columbium surfaces in order to provide the necessary oxidation resistance. These coatings were generally applied by 2. various vapor plating method, that is, by the pack cementation, vapor plating or even fluidized bed processes. All of these processes (hereinafter collectively referred to as cementation processes) consist of a method for subjecting the columbium object to be coated to a reactive or decomposible compounds of silicon and/ or other constituents at elevated temperatures under a controlled atmosphere; the compounds decompose and/ or react on or with the surface of the article to be coated at the temperatures involved. These reactions liberate the coating metals, alloys or intermetallic compound on the surface of the columbium object and, at the elevated temperatures at which these reactions are carried out, the liberated coating diffuses into the columbium surface to produce the desired refractory, oxidation resistant coating.

These known refractory metal coating processes have been moderately successful in attaining the object of applying a uniform, tenacious, adherent, oxidation resistant coating on columbium. However, the nature of these processes and the materials used therein have had certain detrimental effects on the columbium substrate itself resulting in degradation of the properties of the columbium. It is another object of this invention to provide a coating process of which has little or no adverse effect on the columbium substrate.

In the pack cementation coating processes heretofore in use, the article to be coated is packed in a coating mixture which generally consists of the coating metals or a volatile reactive compound thereof (such as, for example the simple metal halides), a carrier compound, an inert filler material (i.e. finely divided aluminum oxide), and an atmosphere control compound capable of forming an inert or reducing atmosphere when heated for producing an atmosphere to protect the objects and other materials during the coating process. Amongst these, the most important is the carrier compound, that is, the compound which will react with the coating metals to produce a reactive compound thereof, which in turn will decompose or react at the surface of the object to be coated.

In the past, atmosphere control compound most commonly used for pack cementation processes was urea (CO(NH) When urea is heated, it decomposes at ap-- proximately 270 F. to form biuret and ammonia. At higher temperatures ammonia further dissociates to a limited extent into nitrogen and hydrogen. All these gases serve to protect the object to be coated and to flush out any residual air or oxygen in the retort containing the packing mixture.

The standard carrier material used in the known processes are the'ammonium halides, particularly ammonium chloride. This compound decomposes at elevated tempera ture forming a hydrogen halide (i.e., or hydrogen chloride) which react with the elementary coating metals such as silicon, molybdenum, tungsten, tantalum, vanadium, boron, columbium or chromium to provide the decomposible or reducible halides of the above metals.

It is known, however, that the materials produced by the decomposition of the atmosphere control and carrier compounds used in the past have an adverse effect on columbium and columbium alloy substrates. In particular, columbium is very prone to absorb hydrogen and is also subject to the formation of nitrides. Both of these problems are aggravated by the use of urea and ammonium salts which dissociate at least partially into hydrogen and nitrogen.

Some attempts have been made in the past to avoid the absorption of hydrogen by using very high processing temperatures, since the occlusion of hydrogen by columbium is exothermic. Unfortunately, high processing temperatures may cause recrystallization of the columbium alloys and impair its mechanical properties. Removal of the hydrogen 'by heat treatment subsequent to the coating process by subjecting the coated objects to high temperatures under vacuum has also been attempted. However, the high temperatures may cause adverse physical effects on the columbium substrate, and the effectiveness of such post heat treatment under vacuum is dependent upon the permability of the coatings to hydrogen. Since most of the coatings are relatively impermeable to hydrogen, the process is not very effective, and therefore, has not found any substantial use in the art.

Attempts have also been made to eliminate the problem of nitrogen embrittlement, without significant success. One approach to this problem has been the substitution of the simple alkali and the alkaline earth halides for the ammonium halides conventionally used for carrier compounds. While the use of these carrier compounds avoided the introduction of nitrogen into the reaction area, the low vapor pressure of the simple alkali and alkaline earth halides rendered it most difficult to obtain a coating of adequate thickness in a reasonable time at a temperature low enough to avoid damage to the columbium alloys.

I have found, however, that the complex fluoro-salts halides have suificient vapor pressure to avoid the difliculties encountered with the simple alkali halides or simsystem. An example of this would be the use of potassiumtantalum fluoride in a coating process using another coating metal, for example, silicon in order to introduce a small controlled amount of tantalum into the silicide coating on the substrate. Other combinations of this sort will be readily apparent to those skilled in the art.

The process described above has been used to deposit silicide type coatings on many different columbium alloys such as those tabulated below, as Well as unalloyed ple alkaline earth halides mentioned above, and at the columbium substrates.

Coluflnbium Composition, Percent by Weight 3, Designation Zr 0 H N G i Hi i Ti Ta W Mo Fe Si V Cb P.p.m., parts per million.

same time, can be effective carriers for the coating materials. In addition, when these compounds break down and form gaseous products they also form an effective protecitve atmosphere and expel residual air from the retort containing the pack mixture. No hydrogen or nitrogen bearing compounds are, therefore, used for atmosphere control reaction, and a superior coating can be produced without damage to the columbium substrate.

FIG. 1 depicts, by flow sheet, an embodiment of the process of this invention.

The process comprising this invention can be carried out with any suitable complex fluoro-salts having suflicient vapor pressure and reactivity to permit the application of a coating in a reasonable amount of time. One salt which has proven to be particularly useful for this purpose is sodium fluosilicate; other complex fluoro-salts compounds such as potassium-columbium fluoride, potassium-tantalum fluoride, potassium-chromium fluoride, a combination of sodium fluosilicate and potassium-columbium fluoride or any other combination of above mentioned complex fluoro-salts carriers can also be used. All of these salts react directly with the metal powders of the pack and/ or the substrate to be coated to produce an oxidation resistant coating on the surface of the substrate via interchange reaction, thermal decomposition, intermetallic compound formation and diffusion processes. Where the complex fluoro-salts are stable, i.e., having relatively low vapor pressure at temperatures involved, these reactions are the only important ones, therefore, they cannot be very effective as an atmosphere controlling compound. Good examples of such stable complex fluorosalts are sodium, lithium or potassium fluoaluminate.

In the case of the unstable or less stable complex fluorosalts, the foregoing reactions take place, and in addition, the decomposition of the carrier complex fluoro compound. During the decomposition of the carrier com pound, non-hydrogen and non-nitrogen bearing reactive components are liberated and take part in the coating reaction with the metal substrate. For example, if potassium-columbium fluoride or potassium-tantalum fluoride are used as carrier compounds, the salts will partially break down below and at the processing temperature and will liberate the columbium or tantalum fluoride, as the case may be, which will enter the aforementioned coating reactions to form the oxidation resistant coating on the substrate. This particular phenomenon can be used for speeding up the coating process, or alternatively, adding another component to form an improved coating While many of these alloys have been coated by the known processes using ammonium halide carriers, the results were not always satisfactory, and two cycle coating processes were often used. These processes comprise a first cycle wherein a layer of a chromium alloy is formed and the second cycle layer is obtained by codeposition of silicon and alloying elements. For some alloys only the two cycle coatings exhibited satisfactory protec tion of the substrate from oxidation. However, certain alloys and in particularly D-36 columbium base alloy, could not be coated at all in pack mixtures containing ammonium halide, a nitrogen bearing carrier. Besides the possible adverse effect of this halide carrier on chemical properties of the alloy substrate, D-36 alloy specimens coated in mixtures containing ammonium halide carrier corroded at the edges and corroded or cracked at the bent or stamped portion of the specimens.

In contrast to the results obtained on D-36 alloy substrate with ammonium halide containing pack mixtures satisfactory results were obtained when ammonium halide carrier was replaced by complex halides and in particular by sodium fluosilic-ate in accordance with this invention. By using these complex fluoro-salts bi-metal fluoro carrier compounds oxidation resistant coatings were deposited successfully in both two cycle and single cycle processes using the D36 alloy as the substrate, and specimens coated in the single cycle process often performed as well as the ones coated with a double cycle coating. Furthermore, in these instances and others, the results were invariably better than the results obtained on coatings which could be applied successfully in a two cycle process with ammonium halide carriers.

This invention can be best understood with reference to a specific example which is intended to be illustrative and not limiting.

Example The process embodying this invention may be carried out to apply an oxidation resistant high temperature coating to a columbium substrate as follows: A mixture is prepared by intimately mixing the following materials in finely powdered form:

Other additions, for example, boron, vanadium or chromium and similar metals, well known to the art, may also be included to achieve desired properties of the coating and to alter the properties of the formed coating.

The object to be coated is cleaned and imbedded in the coating mixture in a closed retort or reaction chamber. Care must be taken to assure that the mixture is in uniform contact with every exposed area of the substrate, in order to assure a uniform supply of coating gases over the entire surface. The charged retort is then sealed, preferably with a mixture of ceramic materials having a melting point slightly below the operating temperature. As the retort is gradually heated up, the ceramic materials fuse, forming a fluid seal which will allow trapped air and other gases to bubble out but which effectively prevent the reentry of air. The retort is held at elevated temperature for a suflicient time for the coating to be formed. The temperatures used in this, process range from 1800 to 2300 F. and the coating period is generally anywhere from 1 to 12 hours, depending on the thickness, the coating desired and the particular materials used.

As the temperature is raised, the complex fluoro salts begin to volatilize and decompose. For example, if sodium fluosilicate (Na SiF is used, this salt will break down into gaseous silicon tetrafluoride and sodium fluoride according to the following equation:

The silicon tetrafluoride and to a much smaller degree the sodium fluoride (due to its high melting point and low vapor pressure at the processing temperature) react with the metal powders of the pack mixture and/ or the substrate to be coated. An oxidation resistant coating is produced then via interchange reaction, intermetallic compounds formation and diffusion processes. Although a variety of associated reactions take placesome of the most important ones can be depicted :as follows:

SiF ]-Me MeF+Si (deposited) SiF +Me- MeF+MeSi (deposited) SiF +MeF MeSi (deposited) where Me denotes the metal powder in the pack mixture and/ or the substrate to be coated. According to the above equations, the silicon deposited on the substrate surface reacts with and diffuses into the base metal to form the desired silicide coating. Furthermore, other metals of the coating mixture i.e. metals powders and/ or complex fluoro-salts carrier are built into the silicide coating to obtain the desired coating system.

If stable complex fluoro-salts carriers such as sodium, lithium or potassium fluoalurninate are used, while substant-ially little or no decomposition occurs at the coating temperatures yet these salts react directly with the reactive metal powder of the pack mixture and with the substrate to be coated. Oxidation resistant coating is produced then as described above. The direct reaction of the stable halide carriers with the metals involved, however, can be slower and less efiicient as compared to the coating process in which unstable complex salt decomposes and its products enter into the reaction. Furthermore, gaseous products of the decomposition of the stable salts do not provide as effective protective atmospheres as the unstable complex halides.

An example of the results obtained by the process herein disclosed as follows:

TABLE II Protective Life (hours) It can be seen that the coatings on both the X- and B-66 alloys provided considerable protection even against cyclic heating to 2600 F. The repeated heating and cooling involved in these tests is a severe test of any coating.

The cyclic oxidation tests were carried out as follows:

several 3 /2 by /2 inch coupons of each of the coated base metals were tested in the atmosphere at temperatures of 1600 F., 2000 F., 2300 F. and 2600 F. All specimens were exposed simultaneously. The test sequence involved cycling the specimens to room temperature once each hour for eight hours followed by sixteen hours of static exposure :at the test temperature in each 24- hour period. Each test was conducted until coating failure oruntil hours exposure without failure. The criteria for coating failure involved visual evidence of coating degradation and/or the growth of columbium oxide from the substrate. 1 The tensile properties of columbium alloys were also affected only very slightly by complex fluoro-salt .temperature treatment the coating process of this invention, as may be seen from the following table:

TAB LE III [Tensile properties of uncoated and single cycle coated D43 alloy at room temperature and 1200 F.]

Room Temperature 1,200 F.

Ultimate 2% of Ultimate 2% of strength, Set Yield strength, Set Yield p.s.i. strength p.s.i. strength,

. p.s.i. p.s.i.

As Received As Received Single Cycle Coated A Single Cycle Coated It should not be construed from the above table that this invention is limited to the application of coatings by the single cycle pack cementation process used for the illustration and examples given above. Any of the other known cementation coating methods such as the vapor plating coating process, fluidized bed process, or any other process operating on the same general principle, alone, or in combination, in single or multiple cycle, may be used. When the word cementation process is used in this application, it is meant to include any of the above processes, or any process hereinafter developed, wherein a coating is produced by the reaction (i.e., decomposition, disproportionation, or other) of a metallic compound in the vapor phase on the surface to be coated.

It should also not be construed that this invention is limited to the coating of substrates of columbium or columbium alloys. The examples given have been all restricted to columbium and columbium alloys because it has been found that the problems caused by hydrogen occlusion and nitrogen embrittlement are most serious in this case. However, these problems also exist in the coating of other refractory or reactive metals and, where such is the case, the coating process herein disclosed is equally applicable. It is, therefore, to be understood that this processes can be used in connection with not only columbium, but also molybdenum, tungsten, tantalum, and any other metals which can be coated by the cementation process.

The complex fluoro salts which may be used are limited only by the absence of nitrogen and hydrogen compounds particularly the ammonium radical. The stable salts may be used, but the unstable salts have the additional property of providing additional metal-s for the cating process and by decomposing to provide protective atmosphere by expelling the air from the coating chamber. Any of the salts can theoretically be used; however, in practice the salt should be limited to those which are readily volatile at temperatures below 2300 F.

It will thus be seen that the invention has attained its stated objects. A simple effective coating means has been provided which totally eliminates the problem of hydrogen occlusion, and nitrogen e-mbrittlement without in any Way detracting from the efficiency of the process. Known techniques, alloys and substrates may be used and a coating of superior properties is produced without in any way degrading or harming the base metal.

While I have shown and described the preferred modes of practice of my invention iit will be apparent that those knowledgeable in the art may make various modifications and changes in recipe compositions, base or substrate metals and reaction conditions and yet fall within the purview of my invention as set forth in the appended claims.

I claim:

1. A process for applying a cementation coating of a reactive metal, said metal selected from the group consisting of silicon, tungsten, molybdenum, boron, vanadium, chromium, tantalum and columbium to a metal substrate, said substrate selected from the group consisting of columbium, molybdenum, tungsten, tantalum and their alloys, said process comprising heating said substrate, in the presence of said reactive metal in particulate form and a particulate complex fluoro salt selected from the class consisting of sodium fluosilicate, potassium columbium fluoride, po-

. tassium tantalum fluoride, potassium chromium fluoride, sodium fi-uoal-uminate, lithium fluoaluminate, potassium fluoaluminate and mixtures thereof, to a temperature within the range of 1 800 F. to 2300 F. in an environment substantially freed of reactive oxygen, hydrogen and nitrogen.

2. A process as in claim 2 wherein said complex fluorosalt is sodium fluosilicate.

3. A process as in claim 1 wherein said reactive metal is a mixture of silicon, tungsten and columbium.

4. A process as in claim 1 wherein said reactive metal is a mixture of silicon, molybdenum and columbium.

5. A process as claimed in claim 1 wherein said complex fluoro-salt is a compound selected from the group consisting of sodium fluosilicate, potassium columbium fluoride, potassium taut-alum fluoride, potassium chromium fluoride, sodium fluoaluminate, lithium fluoaluminate, potassium fluoaluminate and mixtures thereof.

6. A process as claimed in claim 1 wherein said metal substrate is a columbium alloy.

7. A process as claimed in claim 1 wherein said heating is carried out at a temperature of 1800 F. to 2300 F. for a period of one to twelve hours.

8. A process for applying a cementation coating to columbium and columbium alloy articles comprising the steps of packing said articles in a granular mixture comprising a reactive coating metal selected from the group consisting of silicon, tungsten, molybdenium, boron, vanadium, chromium, tantalum and columbium, a complex fluoro salt selected from the group consisting of sodium fluosilicate, potassium columbium fluoride, potassium tantalum fluoride, potassium chromium fluoride, sodium fluoaluminate, lithium fluoaluminate, potassium fiuoaluminate and mixtures thereof, and an inert filler in a reaction chamber, sealing said chamber, and heating said chamber to a temperature of between 1800 F. and 2300 F. for an effective interval to provide said coating.

References Cited UNITED STATES PATENTS 2,689,807 .9/1954 Kempe et a1. 117- 107.2 2,855,332 10/1958 Samuel 117107.2 3,037,883 6/1962 Wachtell et a1 117107.2 3,117,846 1/1964 Chao 1171-07.2 3,249,462 5/1966 Jung et al. l17l06.0

ALFRED L. LEAVITT, Primary Examiner.

A. H. ROSENSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,335 ,028 August 8 1967 Janez Zupan It is hereby Certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

'n the Table, sixth column, line 9 thereof,

Columns 3 and 4 1 for ".92.04" read .02-.04 column 3, line 34, after column 4, lines 40 "control" insert or for the coating and 41, for "chemical" read H mechanical column 6, line 20, for "by" read by the line 21 strike out "the"; column 7 line 37, for the claim reference numeral "2" read 5 Signed and sealed this 24th day of September 1968.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A PROCESS FOR APPLYING A CONCENTRATION COATING OF A REACTIVE METAL, SAID METAL SELECTED FROM THE GROUP CONSISTING OF SILICON, TUNGSTEN, MOLYBEDENUM, BORON, VANADIUM, CHROMIUM, TANTALUM AND COLUMBIUM TO A METAL SUBSTRATE, SAID SUBSTRATE SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, MOLYBDENM, TUNGSTEN, TANTALUM AND THEIR ALLOYS, SAID PROCESS COMPRISING HEATING SAID SUBSTRATE, IN THE PRESENCE OF SAID REACTIVE METAL IN PARTICULATE FORM AND A PARTICULATE COMPLEX FLUORO SALT SELECTED FROM THE CLASS CONSISTING OF SODIUM FLUOSILICATE, POTASSIUM COLUMBIUM FLUORIDE, POTASSIUM TANTALUM FLUORIDE, POTASSIUM CHROMIUM FLUORIDE, SODIUM FLUOALUMINATE, LITHIUM FLUOALUMINATE, POTASSIUM FLUOALUMNATE AND MIXTURES THEREOF, TO A TEMPERATURE WITHIN THE RANGE OF 1800*F. TO 2300*F. IN AN ENVIRONMENT SUBSTANTIALLY FREED OF REACTIVE OXYGEN, HYDROGEN AND NITROGEN. 